The grammatical mistake in the sentence, 'Learning to fly a plane and swimming were two skills Lucy really wanted to learn.' is that its parallel structure is incorrect.
A grammatical mistake is a mistake that takes place in a sentence such that it breaks the set of grammatical rules in maintaining the correct structure of a sentence.
In the above sentence, a grammatical mistake of incorrect use of a parallel structure can be seen, as the skills of flying and swimming are used in different forms of the verb.
Hence, option B holds true regarding the grammatical mistake.
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